Reliable extraction plays a key role in a large number of industrial workflows and the dry bulk solids business is no exception. Here, industry troubleshooter Richard Farnish of the UK’s Greenwich University gives an overview
MANY INDUSTRIAL PROCESSES DEPEND ON THE RELIABLE EXTRACTION OF DRY BULK SOLIDS FROM BUNKERS, SILOS, hoppers or bins. Equally important is the controllability and repeatability of their delivery into down-stream processes. Richard Farnish examines the implications in detail. Many process requirements do not make high demands on the performance of feeders and a degree of fluctuation in output quality - bulk density, size distribution or blend homogeneity - can be tolerated. So ‘standard’ equipment linked to a storage vessel can be adequate. But many industrial operations cannot operate acceptably with “standard” type equipment that has been constructed and interfaced with minimal consideration of the bulk properties of the material being handled. The storage vessel and feeder must be seen as a single entity, in terms of design or trouble-shooting. It is critical that the influence of the flow pattern developing within the storage equipment during discharge is understood, as this will have a major impact on the bulk conditions that occur at the outlet and with which the feeder must interact. The most commonly encountered flow is known as core/funnel flow. Storage schemes with this type of discharge pattern develop a preferential
flow channel
that propagates upwards from the feeder towards the top surface of the inventory. As material drains from this central region the top surface will dip downwards in the middle as material is displaced.
The discharge is sustained by material sloughing off from the top surface and flowing down the preferential flow channel. In some instances, the flow properties of the bulk solid are such that the flow channel can expand above the outlet and intercept with the vertical wall, with the material above appearing to move downwards in an “en masse” over the cone formed by the static region of bulk solids. This is known as an expanded flow or internal mass flow pattern.
BULK DENSITY VARIATIONS The stability of such flow stoppages can vary depending upon the nature of the bulk solid and can range from completely stable, requiring manual intervention to clear, or unstable, in which case ‘flooding’ or ‘flushing’ may occur when the arch or empty flow channel collapse – entraining air and semi-fluidising the bulk solid. Intermittent flow is often associated with core/funnel flow equipment. Another feeder issue concerns variations in bulk density at the outlet. Because a preferential flow channel can be present, the mass of material tends to exit through a constricted flow pathway so the stress acting at the base of the channel is a direct function of the head of material above. As the inventory reduces during a complete discharge, a corresponding reduction in pressure at the base also
CONTROLLED FEEDING
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